Ultraviolet (UV)-emitting phosphors are used in fluorescent lamp applications for skin tanning where both UVA and UVB radiation is needed. UVA is defined by the U.S. Food & Drug administration (FDA) as radiation from 320 nm to 400 nm and UVB is defined as radiation from 260 nm to 320 nm. In general, UVA radiation mainly induces immediate pigmentation. This results in fast tanning and a grayish-brown color of the skin that disappears after a short time. On the other hand, UVB radiation promotes a long-lasting, reddish brown tanning of the skin. However, prolonged exposure to UVB radiation will also result in severe sunburn. Thus, the majority of the UV flux generated by tanning lamps is emitted in the UVA region with the balance in the UVB region.
To provide a fairly good tan, a suntan lamp usually produces a controlled amount of UVB for melanogensis and an amount of UVA sufficient to induce immediate pigment darkening. In the present state of the art, this is achieved by blending different UVA- and UVB-emitting phosphors to yield a proper balance of UVA and UVB, generally intended to mimic the relative proportions of UVA and UVB in natural sunlight. The most common UVA-emitting phosphor is lead-activated barium disilicate (BaSi2O5:Pb). Among all the UVA-emitting phosphors available in the current market, the BaSi2O5:Pb phosphor exhibits the strongest UVA emission under 254 nm excitation with its peak emission occurring at about 350 nm. Therefore, at the present time, the majority of suntan lamps use phosphor blends containing a large proportion of BaSi2O5:Pb as the major UVA-emitting phosphor and a much smaller amount of a UVB-emitting phosphor such as MgSrAl11O17:Ce, LaPO4:Ce, or (Ca,Zn)3(PO4)2:Tl.
However, there are drawbacks to the use of the BaSi2O5:Pb phosphor. One drawback is that like most silicate phosphors the lumen maintenance of the BaSi2O5:Pb phosphor in fluorescent lamps is poor relative to other fluorescent lamp phosphors. In order to improve lumen maintenance, a protective alumina coating is typically applied to the phosphor particles. A preferred method for applying the protective coating to the phosphor particles is via a CVD reaction in a fluidized bed (U.S. Pat. Nos. 5,223,341 and 4,710,674). While effective, this CVD method requires relatively complex coating equipment and hazardous chemicals. Another drawback is the lead activator itself. There is increasing pressure on all manufacturers to eliminate lead from their products because of environmental concerns related to their disposal. Thus, a lead-free, non-silicate alternative to the BaSi2O5:Pb phosphor would offer a significant advantage to lamp manufacturers.
Phosphate phosphors generally achieve higher lumen maintenance levels than silicate, borate, and aluminate phosphors in fluorescent lamps. There are three common cerium-activated orthophosphate phosphors including YPO4:Ce, LaPO4:Ce, and GdPO4:Ce which all give a strong UV emission when excited by 254 nm radiation. The YPO4:Ce phosphor exhibits essentially all UVA emission with two main emission peaks occurring close to 335 nm and 355 nm while LaPO4:Ce displays a strong UVB emission with a major peak at 316 nm and a shoulder peak at 333 nm. The main emission peak of GdPO4:Ce is near 312 nm which is the typical emission found in all Gd3+ containing phosphors. Of these phosphors, YPO4:Ce would be the best choice as an alternative UVA-emitting phosphor in a lead-free suntan lamp. However, the initial UVA output of YPO4:Ce is at least 5% lower than that of BaSi2O5:Pb in a suntan lamp. Thus, it would be an advantage to improve the UVA output of a YPO4:Ce phosphor in order to provide an acceptable alternative to the BaSi2O5:Pb phosphor.